1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the upgrading of a naphtha stream by a combination of reforming with reformate separation and paraffin isomerization.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Such key options as increased reforming severity and higher FCC gasoline octane result in a higher aromatics content of the gasoline pool, through the production of high-octane aromatics at the expense of low-octane heavy paraffins. Current gasolines generally have aromatics contents of about 30% or higher, and may contain more than 40% aromatics.
Currently, refiners are faced with the prospect of supplying reformulated gasoline to meet tightened automotive emission standards. Reformulated gasoline would differ from the existing product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics. The aromatics content may be lowered over several years to a maximum of as low as 20%.
Since aromatics have been the principal source of increased gasoline octanes during the recent lead-reduction program, severe restriction of the aromatics content will present refiners with processing problems. Currently applicable technology includes such costly steps as recycle isomerization of light naphtha and generation of additional light olefins and isobutane as feedstock to an alkylation unit. Increased allowable oxygenates will help, but novel processing technology is needed.